The hermetic seal on shuttle rocket booster (SRB) pressure sensors cannot be easily tested for leaks. Currently, if a leak is suspected in one of these hermetic seals, the seal can be tested only by traditional seal leak evaluation methods, which may require disassembling the transducer or some other form of destructive testing. With SRB pressure sensors in limited supply, disassembly or destructive testing is not favorable.
Other instruments, as well as consumer products (e.g., vitamins, milk, etc.), contain hermetic seals that may need to be tested for leaks. Presently, most seals are tested by introducing a foreign gas, such as helium. A hermetically sealed device is pressurized under vacuum conditions with helium for a period of time inside a vacuum tight chamber. If there is a leak in the seal, helium is forced into the sealed device. The vacuum chamber is then vented. The helium is then pulled out of the vacuum tight chamber, by vacuum pump, and a slight vacuum is pulled on the unit being tested. Any helium that was forced into the device as a result of a leaking seal will escape from the device in the lower pressure. A mass spectrometer is used to detect helium leaking from inside the hermetic seal under vacuum conditions.
This pressurized gas means of leak detection is problematic in many respects. First, it cannot be used to detect large leaks. When the pressurized helium (or other gas) is removed by vacuum pump, any helium escaping from a large seal leak will similarly be removed and no helium will show up on the mass spectrometer, resulting in a false reading that the seal is good. Second, the method requires the removal of gas permeable materials (e.g., soft components such as gaskets and grommets). Deconstruction of these devices, particularly SRB pressure sensors, to remove gas permeable materials is risky as gaskets and grommets can break or tear easily during removal. This method is also time-consuming, requiring multiple sessions of pressurizing, depressurizing, and observing. Further, there is the risk of contamination with whatever gas is chosen. Finally, mass spectrometers are very expensive, making this pressurized gas system of hermetic seal leak detection very expensive. Approximately $10,000 of hardware is needed for this method of testing.
Hermetic seals are alternatively tested by pressurizing the device being tested and then submerging it in a chemical bath. If there is a hole in the device, gas will enter the device when it is pressurized and escape into the lower pressure area when submerged. The escaping gas creates bubbles, which can be visually observed.
This pressure/submersion means of leak detection is also problematic in many of the same ways as the pressurized gas means of leak detection. First, all soft goods (e.g., gaskets and grommets) must be removed, leading to the same deconstruction problems observed with the pressurized gas means of leak detection. Second, if there is a leak in the device, there is the risk of contamination with the chemical when the device is submerged. This is particularly important for SRB pressure sensors, which cannot have moisture buildup or condensation on the interior of the sensor. This method is also time-consuming, requiring multiple steps and long periods of observation. Finally, this pressure/submersion means of leak detection requires not only vacuum chamber and pump devices, but also a chemical bath, making it just as costly as the pressurized gas means of leak detection.
It is desirable to have a simple, low-cost system for detecting hermetic seal leaks.
It is desirable to have a system for detecting hermetic seal leaks that eliminates the need for pressurizing gas.
It is desirable to have a system for detecting hermetic seal leaks that does not require disassembly or destruction of the unit/instrument before testing for a leak.
It is desirable to have a system for detecting hermetic seal leaks that is capable of detecting very small leaks, as well as large leaks.
It is desirable to have a system for detecting hermetic seal leaks that does not require the removal of gas permeable materials.
It is further desirable to have a system for detecting hermetic seal leaks which does not contaminate the unit being tested with helium or other detection gases or liquids.